1. Field of the Invention
The present invention relates generally to a method of fabricating an optical waveguide device and in particular, to a method of forming a waveguide included in an integrated optical circuit or a planar lightwave circuit (PLC) used in an optical communication system.
2. Description of the Related Art
An integrated optical circuit typically includes a linear or curve shaped optical waveguide, which is designed to propagate light from one point to another or filter the light by controlling the wavelength characteristic of the light, a mirror, and a lens. The optical waveguide for controlling a refractive index to silica (SiO2) and obtained by adding dopants, such as GeO2, P2O5 and B2O3, is popularly used for fabricating the integrated optical circuit.
In attempts to increase the efficiency of an optical communication system and lower the system costs, it is very important to decrease coupling losses between the optical waveguide and a laser, and between the optical waveguide and an optical fiber in the integrated optical circuit. Various methods to reduce these coupling losses have been suggested. In particular, for a typical single mode optical fiber, since a difference between the indices of refraction of a core and a cladding is fixed to 0.3%, if a high refractive index material is used for fabricating the optical waveguide to reduce the size of the integrated optical circuit, the coupling loss largely increases due to a mode discord between the optical fiber and the optical waveguide. Thus, for fabricating a highly integrated optical device, it is a very important to improve the coupling loss.
For a planar hybrid integration device, it is very important to minimize a coupling loss between a planar hybrid integration device and an optical fiber, and a coupling loss between each unit device and an optical waveguide.
Currently, the most efficient technique to decrease a coupling loss between an optical device and an optical fiber among techniques utilizes an optical mode converter. A mode size of an optical waveguide is similar to a mode size of an optical fiber by changing a cross section of the optical waveguide in a horizontal direction and/or in vertical direction of the substrate of an integrated optical device. A change in a cross section size of the optical waveguide in the horizontal direction of the substrate, i.e., a lateral taper side of the optical waveguide, can be easily obtained by controlling a width of a slit on an etching mask generally used for an optical waveguide fabricating process. However, a change in a cross section size of the optical waveguide in the vertical direction of the substrate, i.e., a vertical taper side of the optical waveguide, requires a more complicated process.
FIG. 1 is a conceptual diagram of an apparatus for forming a planar lightwave circuit (PLC) 110 having a thermally expanded core according to a prior art.
As shown, the apparatus includes a heater 120 and a holder 130. The PLC 110 includes a core 112 and a cladding 114, which are made from silica and dopants. A first edge of the PLC 110 is fixed to the holder 130, and a second edge is inserted into the heater 120 for thermal diffusion. The dopants are diffused from the core 112 to the cladding 114 by heat provided by the heater 120. Accordingly, the size of a cross section of a heated portion of the core 112 is actually expanded.
The heater 120 induces diffusion of the dopants in the core 112 by heating the second edge of the PLC 110.
The holder 130 fixes the PLC 110 by clamping the first edge of the PLC 110.
That is, the dopants are diffused from the core 112 to the cladding 114 by the heat applied from the heater 120, and as a result, the size of a cross section of the heated portion of the core 112 increases. Accordingly, a mode of the core 112 is matched to a mode of an external optical fiber.
The method of fabricating the PLC110 using the heat diffusion described above is known as a chip unit process, and not a wafer unit process, due to heated characteristics of the fabricated PLC 110. Thus, much time and efforts are required for the fabricating using the above process. In addition, since the length of an optical mode converter is very long, about 4 mm, due to a temperature gradient according to the heat diffusion, it is very difficult to implement the above fabricating method.
FIGS. 2 to 4 are sectional diagrams illustrating a method of fabricating a three-dimensionally tapered optical waveguide according to a prior art. The fabricating method indicates a gray scale lithography scheme generally used in the present.
The fabricating method includes the steps of (a) to (c) described below.
Referring to FIG. 2, the step (a) is a process of sequentially layering a silica layer 22 and a photoresist layer 23 on a substrate 21 and deploying a gray scale mask 24 on the photoresist layer 23.
To form the silica layer 22 on the substrate 21, a chemical vapor deposition (CVD) process and a flame hydrolysis deposition (FHD) process may be used, wherein the CVD can be classified into plasma enhanced chemical vapor deposition (PECVD) and low pressure chemical vapor deposition (LPCVD). The photoresist layer 23 can be formed on the silica layer 22 with a predetermined thickness by dropping liquid photoresist on the silica layer 22 and rotating the substrate 21 at a high speed.
The gray scale mask 24, which is deployed on the photoresist layer 23 and fabricated to have different permeability of ultraviolet rays in a horizontal direction, may be a glass material.
Referring to FIG. 3, the step (b) is a process of forming a photoresist mask 33 by photolithographing the photoresist layer 23 using the gray scale mask 24. That is, the photoresist mask 33 is formed through a photo-exposing and developing process using ultraviolet rays. A height of the photoresist mask 33 remaining through the photo-exposing and developing process is changed along the horizontal direction by changing the intensity of the ultraviolet beam radiated on the photoresist layer 23 in the horizontal direction using the gray scale mask 24.
Referring to FIG. 4, the step (c) is a process of forming a three-dimensionally tapered optical waveguide by etching the silica layer 22 using the photoresist mask 33. That is, a vertical profile along the horizontal direction of the photoresist mask 33 is transferred to the silica layer 22 by etching the silica layer 22 using the photoresist mask 33. Accordingly, the three-dimensionally tapered optical waveguide 42 is formed. The gray scale lithography scheme is popularly used for fabricating having three-dimensionally tapered cores and micro lenses.
FIG. 5 is a picture obtained by photographing an example of a three-dimensionally tapered optical waveguide, which is formed by the above-described fabricating method.
However, the biggest drawback of the gray scale lithography scheme is that it is not easy to control the process due to various factors. According to the experimental data in the ultraviolet rays photo-exposing and developing process using the gray scale mask 24, the gray scale lithography scheme is very sensitive to uniformity of a photoresist, air temperature and humidity, temperature of developer, and intensity and time of the ultraviolet rays in the photo-exposing process. Accordingly, the operation control is not easy, and reproducibility is low. In addition, since the gray scale mask 24 is more expensive than a general etching mask, additional expenses are required.